Wrong loop cuts position offered by blender by default
The loop cuts not going all the way around would be the result of the loop cut running into a face that has more or less than 4 sides; an n-gon.

To make a loop cut go in the direction you want, make sure to place your cursor on an edge perpendicular to the direction of the loopcut (cursor is where 3D Cursor is):





These are the basic principles for all loop cut making.